Riser-rated optical cable

ABSTRACT

A fiber optic cable suitable for indoor applications includes a core tube surrounding a plurality of coated optical fibers; a jacket formed of flame-retardant polymer material surrounding the core tube; and at least one layer of strength members disposed between said core tube and said jacket. The jacket has an outer diameter of not greater than about seven mm and the coated optical fibers experience a short-term increase in signal attenuation of no more than about 0.01 dB when the cable is looped in a radius of 5 centimeters.

BACKGROUND OF THE INVENTION

This invention relates to optical fiber cables which are suitable foruse within building vertical shafts and also are provided withprotection against moisture internal migration. In particular, thisinvention relates to single-tube design optical fiber cables suitablefor such uses.

Drop cables are outdoor cables which bring telephone service tobuildings, whereas riser cables provide telephone service withinbuildings. Riser cables extend upwards from basement vaults to wiringclosets located on upper floors.

Drop cables must be able to withstand the changing conditions found inthe outdoor environment. Most materials increase in length and volumewith increases in temperature and decrease in length and volume withdecreases in temperature. Each material may have a different rate ofchange of length given a specified change in temperature. Such a rate iscalled the coefficient of thermal expansion for a material. Becausedifferent materials in a cable may have different coefficients ofthermal expansion, temperature changes may induce strains in the cablecomponents. For this reason, changes in optical fiber attenuation overdifferent temperatures are measured in cables intended for outdoor use.Successful cables must not experience unacceptable increases in opticalfiber attenuation caused by cable strains induced by temperature-relatedconditions.

Drop cables also must be protected against migration of moisture withinthe cable. Although cable jackets are intended to prevent the ingress ofwater into the cable, no plastic material perfectly stops the ingress ofmoisture. Furthermore, water may enter a cable at points where the cablejacket has been damaged, or at the end of the cable. Therefore,longitudinal movement of water along the inside of the cable must beprevented. For this reason, water-blocking or water-absorptive materialis provided in cable interstices which otherwise could act as conduitsfor moisture internal migration. Types of materials which may be usedfor this purpose are gel-like filling and flooding compounds. Fillingcompounds are disposed alongside the optical fibers within buffer tubes,while flooding compounds are disposed in spaces between the cable jacketand the buffer tubes holding the optical fibers. Many filling andflooding compounds are oil or grease-based. As a result, most fillingand flooding compounds provide fuel for combustion. However, most cablesintended for outdoor use are not required to be flame-retardant.

Other types of materials are becoming more widely used in outdoor usecables for protection against cable moisture migration. Examples includewater-absorptive polymers, which may be inserted into a cable as loosepowders or incorporated into tapes which are wrapped about other cablecomponents. Another example is water blocking strength members, asdisclosed in U.S. Pat. Nos. 4,913,517 and 5,389,442.

Cables intended for use within buildings normally are not exposed to themoisture and extreme temperature conditions experienced by cablesintended for outdoor use. However, building cables are required by theNational Electrical Code to meet criteria indicating that the cableswill not act to spread fires within a building. The most well-known teststandard for riser-rated cables is Underwriters Laboratories (UL)Standard 1666, "Test for flame propagation height of electrical andOptical-fiber Cables installed vertically in Shafts (Second Edition,Jan. 22, 1991). The second edition of this standard is referred toherein as UL Standard 1666.

An optical fiber service cable designed to be suitable for both indoorand outdoor use is disclosed in U.S. Pat. No. 5,566,266, which issued onOct. 15, 1996 in the names of Nave and McDowell. However, the disclosedcable is designed for use with a rather high optical fiber count anddiscloses an inner tube which itself has an outer diameter of 10.2 mm.Such a cable could not be connectorized using standard buffer tubefanout kits. The cable also employs a tape formed from materials such asa polyimide. Such tapes significantly add to the cost of the cable, andit is necessary to process and splice such tapes.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide ariser-rated cable having a small diameter and low minimum bend radiuswhich also is formed using low-cost materials.

These and other objects are provided, according to the presentinvention, by a fiber optic cable suitable for both outdoor and indoorapplications, comprising: a core tube surrounding a plurality of coatedoptical fibers; a jacket formed of UV-resistant flame-retardant polymermaterial surrounding said core tube; and at least one layer of strengthmembers disposed between said core tube and said jacket, characterizedin that said jacket has an outer diameter of not greater than aboutseven mm and said coated optical fibers experience a short-term increasein signal attenuation of no more than about 0.01 dB when the cable islooped in a radius of 5 centimeters.

In a preferred embodiment, two layers of strength members are wrappedaround the core tube in opposite directions and the set of two strengthmember layers is disposed between and directly contiguous to said coretube and said jacket. The cable is capable of meeting the flameretardance requirements set out in UL Standard 1666 in the absence of aflame-resistant tape. The strength members may be impregnated with awater blocking material.

The cable may have an average weight not exceeding 53 kg/km while beingcapable of withstanding a short-term tensile load of 1320N.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the invention are described in the severaldrawings, in which:

FIG. 1 is a cut-back perspective view of a cable according to apreferred embodiment; and,

FIG. 2 is a cross-sectional view of the cable of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described more fully hereinafter withreference to the accompanying drawings, in which one or more preferredembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that the disclosure will fully convey the scope of theinvention to those skilled in the art. Like numbers refer to likeelements throughout. The drawings are not necessarily drawn to scale butare configured to clearly illustrate the invention.

A cable 10 according to a preferred embodiment is shown in FIGS. 1 and2. Cable 10 is designed to include from two to twelve coated and colored250 μm optical fibers 11. Optical fibers 11 may be either single-mode ormultimode fibers, or a mixture of single-mode and multimode fibers. Coretube 12 also may contain a filling compound 17 disposed in the spacewithin the core tube not occupied by the optical fibers. The opticalfibers 11 typically are not stranded with each other, and have a lengthwhich is from 0% to 0.2% greater than the length of core tube 12. Coretube 12 itself may be formed from a flexible plastic material such aspolypropylene. In the preferred embodiment, the core tube 12 need not beformed from flame-retardant material.

Core tube 12 may have an inner diameter of 1.8 mm and an outer diameterof 3.0 mm, and loosely contains the optical fibers 11. Buffer tubeshaving an outer diameter of 3.0 mm are widely used, so that the buffertube may be connectorized using equipment which is already available tothe industry.

An inside layer 13 and an outside layer 14 of flexible strength membersare stranded in opposite directions about core tube 12. In a cableaccording to the preferred embodiment, eight yarns form outside layer 14and six yarns form inside layer 13. Inside strength member layer 13 iscontiguous to core tube 12; outside strength member layer 14 is directlycontiguous to inside strength member layer 13; and jacket 16 is directlycontiguous to outside strength member layer 13. A polyester ripcord 15lies at the inner surface of jacket 16.

The strength members forming layers 13 and 14 are chosen to be highlyflexible. In a preferred embodiment, the strength members forming layers13 and 14 may be Advantex™ reinforcements, available from Owens Corning,which are fiberglass yarns including a superabsorbent polymer. Thereinforcements swell up to five times their own weight in deionizedwater, providing effective water-blocking protection to the spacebetween jacket 16 and buffer tube 12.

These strength members are chosen to provide sufficient anti-bucklingand tensile strength to the cable. The Advantex™ reinforcements have atensile modulus of elasticity of 7×10⁴ MPa, allowing the cable accordingto the preferred embodiment to have a maximum tensile loading duringinstallation of 1320N, and a long term maximum tensile load of 330N.

The outer jacket 16 may be formed from polyvinyl chloride material whichis both ultraviolet resistant and flame retardant, adapting the cableaccording to the preferred embodiment for both indoor and outdoor use.The average outer diameter of the outer jacket 16 may be 7.0 mm. Thismakes the cable according to the preferred embodiment small enough to beplaced in existing ducts where space is at a premium.

A sample cable having a length of 20 m containing three multimode fibersand nine single-mode fibers was tested for optical fiber attenuation atlow bend radius. The cable excess fiber length percentage was 0.2%. Thesingle-mode fibers were concatenated and terminated separately from themultimode fibers. Attenuation test sets operating at 1300 nm formultimode fibers and 1550 nm for single-mode fibers were connected tothe concatenated fibers. The cable sample was placed in a loop using atemplate with a possible radii range of 8 cm to 2 cm. The loop diameterwas slowly decreased from 8 cm down to 2 cm while the opticalattenuation (ΔdB) was measured. Table 1sets out the results. (Results at2 cm are not reproduced , as the optical fiber itself has a minimum bendradius of about 2.5 cm for long-term mechanical reliability). Due to thenature of the test, single-mode values are to be divided by nine, andmultimode values are to be divided by 3 to obtain average values forindividual optical fibers.

                  TABLE 1                                                         ______________________________________                                        Change in attenuation (Δ dB) in concatenated optical fibers at          different cable bend radii; results given separately for                      single-mode (SM) and multimode (MM)                                           Bend Radius                                                                            8 cm   7 cm     6 cm 5 cm   4 cm 3 cm                                ______________________________________                                        Trial 1 SM                                                                             0.00   0.00     0.02 0.02   0.04 0.09                                Trial 1 MM                                                                             0.00   0.01     0.01 0.02   0.04 0.07                                Trial 2 SM                                                                             0.00   0.00     0.00 0.01   0.03 0.03                                Trial 2 MM                                                                             0.00   0.00     0.01 0.03   0.05 0.05                                Trial 3 SM                                                                             0.00   0.01     0.01 0.01   0.02 0.03                                Trial 3 MM                                                                             0.00   0.01     0.01 0.01   0.02 0.05                                ______________________________________                                    

When the proper division is made, it is seen that both the single-modeand the multimode individual optical fibers had a change of signalattenuation of 0.01 dB or less at a cable bend radius of 5 cm.Furthermore, the cable did not kink even at a bend radius of 2 cm.

The cable according to the preferred embodiment performs very wellduring stripping, handling, and bending. Because of the small outsidediameter and flexibility of the cable, a ring cut is difficult to makein the jacket using a hook blade. Use of a straight blade for thispurpose therefore is recommended.

Cables according to the preferred embodiment may be used ininterbuilding and intrabuilding backbones in aerial, duct, or riserapplications. These cables have a specified operating temperature of-40° C. to +70° C. These cables are UL 1666 listed and meet therequirements of ICEA-596.

The cable core comprising tube 12, filling compound 17 and coatedoptical fibers 11 may be made using either a vertical or horizontalbuffering line as known to the prior art. Spinners may be used to applystrength reinforcement member layers 13 and 14. The tension applied tothe strength members may be 350 g, and their lay length may be 250 mm.In jacketing cables according to the preferred embodiment, a tipdiameter of 5.25 mm and a die diameter of 7.0 mm may be used. Six inchesmay separate the die orifice and a cooling water vat, and the extrudertemperature profile used in forming the outer jacket may cover the range142°-185° C. A line speed of 25 m/min. may be employed.

Aramid fiber yarns coated with a swellable powder or film arealternative strength members which may be used. A flame-retardantpolyethylene material may be used as a jacket material for zero halogen,low smoke applications.

It is to be understood that the invention is not limited to the exactdetails of the construction, operation, exact materials, or embodimentsshown and described, as modifications and equivalents will be apparentto one skilled in the art without departing from the scope of theinvention.

What is claimed is:
 1. A fiber optic cable suitable for indoorapplications, comprising:a core tube surrounding a plurality of coatedoptical fibers; a jacket formed of flame-retardant polymer materialsurrounding said core tube; at least one layer of strength membersdisposed between said core tube and said jacket, said strength membersperform an anti-buckling function in said fiber optic cable; said jackethas an outer diameter of not greater than about seven millimeters; andsaid strength members preventing buckling of said fiber optic cablewhereby said coated optical fibers experience a short term increase inattenuation of no more than about 0.01 dB when the cable is looped in aradius of five centimeters.
 2. A fiber optic cable as set out in claim1, wherein said core tube and said jacket each are contiguous to saidstrength members.
 3. A fiber optic cable as set out in claim 1, whereinsaid cable meets the flame retardance requirements set out in ULStandard 1666 in the absence of a flame-resistant tape.
 4. A fiber opticcable as set out in claim 1, wherein said strength members comprisewater blocking material.
 5. A fiber optic cable as set out in claim 1,comprising two layers of said strength members wrapped about said coretube in opposite directions.
 6. A fiber optic cable as set out in claim1, having a weight not exceeding 53 kg/km while being capable ofwithstanding a short-term tensile load of 1320N.
 7. A fiber optic cableas set out in claim 1, further comprising a filling compound disposed inthe space within the core tube not occupied by the optical fibers.
 8. Afiber optic cable, comprising:a cable jacket having an outside diameterof about 7 millimeters or less; a core tube, said core tube having atleast one optical fiber therein; and strength members, said strengthmembers generally encircling said core tube, at least one of saidstrength members being contiguous with said core tube, being contiguouswith said jacket, and being contiguous with at least one other of saidstrength members for minimizing attenuation in said at least one opticalfiber.
 9. The fiber optic cable of claim 8, wherein said at least one ofsaid strength members that is contiguous with said core tube, iscontiguous with at least two of said strength members for minimizingattenuation in said at least one optical fiber.
 10. A fiber optic cable,comprising:a cable jacket having an outside diameter of about 7millimeters or less; a core tube, said core tube having at least oneoptical fiber therein; and strength members, said strength members beingarranged in a generally encircling, staggered pattern about said coretube wherein a first layer of said strength members is generallyarranged so that strength members of the first layer are in contact withsaid core tube, and wherein a second layer of said strength members isgenerally arranged so that strength members of the second layer are incontact with said jacket but are not in contact with said core tube. 11.The fiber optic cable of claim 10, wherein said first layer of strengthmembers is stranded in a direction of lay is substantially opposite tothe direction of lay of said second layer of strength members.
 12. Thefiber optic cable of claim 10, wherein said first and second layerscomprise strength members that are oblong in cross section.
 13. Thefiber optic cable of claim 10, wherein at least some of said strengthmembers are impregnated with a dry water absorbent substance.